Medical devices, such as cardiac pacemakers and ICDs, provide therapeutic electrical stimulation to a heart of a patient via electrodes carried by one or more medical electrical leads and/or electrodes on a housing of the medical device. The electrical stimulation may include signals such as pacing pulses or cardioversion or defibrillation shocks. In some cases, a medical device may sense cardiac electrical signals attendant to the intrinsic or pacing-evoked depolarizations of the heart and control delivery of stimulation signals to the heart based on sensed cardiac electrical signals. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation signal or signals may be delivered to restore or maintain a more normal rhythm of the heart. For example, an ICD may deliver pacing pulses to the heart of the patient upon detecting bradycardia or tachycardia or deliver cardioversion or defibrillation shocks to the heart upon detecting tachycardia or fibrillation. An extra-cardiovascular ICD system utilizes therapy delivery electrodes located outside the cardiovascular system, which avoids having to introduce implantable leads and electrodes within the patient's bloodstream. Electrical stimulation therapies that are delivered using extra-cardiovascular electrodes may require higher voltages in order to be effective compared to electrical stimulation therapies delivered using electrodes proximate to or in intimate contact with cardiac tissue, such as endocardial electrodes or epicardial electrodes.